3D Thrust sheet continuity in Assynt

You will need:

  1. Geological map of Assynt
  2. Simplified map of Assynt. (click here for pdf)

This exercise is designed to allow you to use 3D aspects of thrust geometry, particularly branch lines and cut-off lines, to examine structural interpretations. The base map is the geological 'tourist' sheet to Assynt, part of the Moine thrust belt of NW Scotland, drawn from geological surveys at the end of the 19th century. In common with all maps, it is an interpretation of the geology. Treat correlations of different thrust segments (e.g. the Ben More thrust) with caution. Thrust aren't labelled in the field! You are provided with a simplified tracing which will allow you to mark on branch lines and cut-off lines. However, to complete the exercise and to find the thrusts as interpreted by the original Survey geologists, you will need to use the original map. (Moine thrust = T'''; Ben More thrust = T''; Glencoul Thrust = T' on this map).

A web-solution for this exercise is available.

The following analytical strategy is proposed:

  1. Construct the branch line for the Ben More and Moine thrusts. You should first identify all the branch points and then join them, testing whether the emergent-buried behaviour is maintained around the complete loop.
  2. Construct the hanging-wall cut-off lines for the base of the Cambrian against the Ben More thrust.
  3. Sketch a stratigraphic separation diagram for the Ben More thrust between a and b. In theory this should involve first drawing all the cut-off maps for footwall and hanging-wall. But it can be sketched now by projecting information (fw and hw) from the main trace of the thrust.
    Comment on the different stratigraphic relationships along the Ben More thrust.
  4. Now armed with the geometric interpretation, check the geometry of the Ben More sheet. Does it behave like a thrust sheet (older on younger, thrust cutting simply up-section in transport)? If not, suggest modifications in the map interpretation or suggest a structural history that explains the relationships that you have found.

The original map, published by the British Geological Survey, is the Special Sheet to the Assynt district (1:63360), originally published in 1923 but reprinted in 1986.

This exercise is discussed, in part, by the following articles:

Elliott, D. & Johnson, M.R.W. 1980. Structural evolution in the northern part of the Moine thrust belt, NW Scotland. Trans. R. Soc. Edinburgh 71, 69-96.

Coward, M.P. 1985. The thrust structures of southern Assynt, Moine thrust zone. Geological Magazine 122, 595-607.